Navigated transcranial magnetic stimulation (TMS) is a tool for targeted, noninvasive stimulation of cerebral cortex. Transcranial stimuli can depolarize neurons and evoke measurable effects which are unique in two ways: the effects are caused directly and without a consciousness of the subject, and, the responses from peripheral muscles provide a direct measure for the integrity of the whole motor pathway. The clinical relevance of the method has not always been fully exposed because localizing the optimal stimulation site and determining the optimal stimulation strength have been dependent on time-consuming experimentation and skill. Moreover, in many disorders it has been uncertain, whether the lack of motor responses is the result of true pathophysiological changes or merely because of unoptimal stimulation. We characterized the muscle responses from human primary motor cortex system by navigated TMS to provide normative values for the clinically relevant TMS parameters on 65 healthy volunteers aged 22 to 81 years. We delivered focal TMS pulses on the primary motor area (M1) and recorded muscle responses on thenar and anterior tibial muscles. Motor threshold, latencies and amplitudes of motor-evoked potentials, and silent period duration were measured. The correction of the motor-evoked potential latency for subjects' height is provided. In conclusion, we provide a modified baseline of TMS-related parameters for healthy subjects. Earlier such large-scale baseline material has not been available.
"First, the primary motor cortex on the left hemisphere was mapped for the optimal stimulus site for the abductor pollicis brevis (APB) muscle using nTMS (Säisänen et al., 2008a). Prior to cSP measurements, resting motor threshold (rMT) was measured for the right hand (Säisänen et al., 2008b). "
[Show abstract][Hide abstract] ABSTRACT: Cortical silent period (cSP) is a short interruption in electromyography (EMG) during active muscle contraction induced with transcranial magnetic stimulation (TMS). The cSP is a measure of cortical inhibition and is believed to represent inhibitory interneuron effects on excited motor cortical areas. Several pathological conditions and pharmacological manipulations induce changes to cSP duration indicating alterations in intracortical inhibition. At present, it is common to manually analyse the cSP duration from measured EMG. However, to avoid inter-examiner effects on cSP interpretation and detection, as well as to allow for quick measurement online, automated routine would be preferable. In this study, we evaluate the feasibility of a straight-forward cSP detection routine based on analysing the rectified first derivative of the EMG signal following TMS. Previously measured cSPs of 54 healthy subjects were reanalysed manually by two of the authors and using the automated routine. Furthermore, we recruited one subject for whom the cSPs were induced with several stimulation intensities, and those cSPs were analysed manually by two of the authors as well as using the automated routine. We found that cSPs were detected correctly by the automated cSP detection routine, and agreement with manually analysed subject-specific mean cSPs was excellent (ICC=0.992, p<0.001). The inter-examiner variability was similar to the variability between manual and automated analysis. Hence, we believe the introduced cSP detection routine would be feasible for online cSP detection, in such a way that is presently used to detect the motor evoked potentials.
"To avoid false positive registrations of the nTMS-induced MEPs (e.g., due to direct stimulation of the facial nerve), we only accepted latencies within the following ranges for the respective group of muscles: 17–27 ms for APB, 36–50 ms for PM, and 7–15 ms for MM as well as for MEP recordings of the LT (Muellbacher et al., 2001; Rodel et al., 1999, 2001, 2003; Saisanen et al., 2008). The order of stimulation (APB, PM, MM, LT) was randomized across subjects. "
[Show abstract][Hide abstract] ABSTRACT: Introduction:
Functional magnetic resonance imaging (fMRI) is a frequently used non-invasive mapping technique for investigating the human motor system. Recently, neuronavigated transcranial magnetic stimulation (nTMS) has been established as an alternative approach. We here compared the test-retest reliability of both mapping techniques with regard to the cortical representations of the hand, leg, face and tongue areas.
Ten healthy subjects were examined three times (intervals: 3-5days/21-35days) with fMRI and nTMS. Motor-evoked potentials were recorded from the abductor pollicis brevis, plantaris, mentalis and the tongue muscles. The same muscles were activated in an fMRI motor task. Euclidean distances (ED) between hotspots and centers of gravity (CoG) were computed for the respective somatotopic representations. Furthermore, spatial reliability was tested by intersession overlap volumes (OV) and voxel-wise intraclass correlations (ICC).
Feasibility of fMRI was 100% for all body parts and sessions. In contrast, nTMS was feasible in all sessions and subjects only for the hand area, while mappings of the foot (90%), face (70%) and tongue representations (40%) remained incomplete in several subjects due to technical constraints and co-stimulation artifacts. On average, the mean ED of the hotspots was better for fMRI (6.2±1.1mm) compared to nTMS (10.8±1.9mm) while stability of CoG was similar for both methods. Peak voxel reliability (ICC) was high for both methods (>0.8), and there was no influence of inter-session intervals. In contrast, the reliability of mapping the spatial extent of the hand, foot, lips and tongue representations was poor to moderate for both fMRI and nTMS (OVs and ICC<50%). Especially nTMS mappings of the face and tongue areas yielded poor reliability estimates.
Both methods are highly reliable when mapping the core region of a given target muscle, especially for the hand representation area. In contrast, mapping the spatial extent of a cortical representation area was only little reliable for both nTMS and fMRI. In summary, fMRI was better suited when mapping motor representations of the head, while nTMS showed equal reliability for mapping the hand and foot representation areas. Hence, both methods may well complement each other.
"The findings of the present study indicate that the popular OM method, used for determining TMS MT, is a simple, reliable and viable alternative to the highly variable (Kiers et al., 1993; Säisänen et al., 2008; Jung et al., 2010) EMG-assisted method. "
[Show abstract][Hide abstract] ABSTRACT: The aim of this study was to establish the reliability of the observation of movement (OM) method for obtaining motor threshold (MT) in transcranial magnetic stimulation (TMS). MTs were obtained on separate days, following separate hunting procedures, for both left and right motor cortex (M1), with one or multiple estimates obtained from the same hemisphere within a single session. MTs obtained using the OM method were highly reliable and reproducible on different days (left M1: r=.98, p<.0001; right M1: r=.97, p<.0001). MTs were not influenced by the order of acquisition when two hemispheres were stimulated in the same session [F(1,22)=.12, p=.73] or by the collection of additional MTs as part of the distance-adjusted procedure [F(1,23)=.74, p=.40]. The results verify the reliability of the OM method and confirm its viability for the safe and efficient application of TMS to the left and right M1. The OM method is a reliable technique for obtaining MT and is relatively simple and quick to run. It therefore provides an effective procedure for research and clinical applications.
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